This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
In the process of electrostatographic printing, electrostatic charge patterns are formed and reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent charge pattern. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent charge pattern. Development, which is the act of rendering an electrostatic latent pattern or image visible, is employed in all of the aforementioned types of electrostatography. Hereinafter, an electrophotographic process will be described as the illustrative embodiment having the features of the present invention incorporated therein.
An electrophotographic printing machine employs a photosensitive element having a photoconductive insulating layer which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged photoconductive surface is exposed to a light image of an original document being reproduced. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the surface thereof. This records an electrostatic latent image on the photoconductive surface. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix adjacent thereto. A typical developer mix employs dyed or colored plastic particles, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. In general, the toner particles are heat settable. The toner particles and carrier granules are selected so that the toner particles have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is adjacent the photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes some of the toner particles to transfer from the carrier granules and adhere thereto. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
In electrophotographic printing, excess developer mix is positioned adjacent the electrostatic latent image. The unused developer mix and denuded carrier granules are returned to the sump thereof for subsequent reuse. Mixing of the unused developer mix and denuded carrier granules with new toner particles is promoted by cross mixing the foregoing with one another. This insures that the triboelectric characteristics will be satisfactory. However, hereinbefore it has been found that the developer mix and unused carrier granules frequently clog or block the baffle preventing cross mixing. This may produce an erroneous indication of there being insufficient developer mix within the printing machine.
Accordingly, it is the primary object of the present invention to improve the development system by having cross mixing of the unused developer mix and denuded carrier granules without developer mix clogging.